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| United States Patent |
5,703,195
|
|
Schultz
, et al.
|
December 30, 1997
|
Polyglycidylphenyl ethers of alkylene or alkyleneoxy chains for use in
microelectronics adhesives
Abstract
This invention relates to flexible epoxy resins characterized by a low
ionic contamination, total chlorine content less than 0.1%. The epoxy
resins are liquids at room temperature, have Tg values when cured of less
than or equal to 150.degree. C., have neat resin viscosities of 25,000 cps
or less at 25.degree. C., and have a structural composition comprising an
oligomeric backbone of alkylene repeat units, terminated with an aromatic
moiety bearing one or more epoxy functionalities. The flexible epoxy
resins are used in adhesives for microelectronic applications.
| Inventors:
|
Schultz; Rose Ann (Princeton, NJ);
Fenelli; Steven P. (Hillsborough, NJ)
|
| Assignee:
|
National Starch and Chemical Investment Holding Corporation (Wilmington, DE)
|
| Appl. No.:
|
757187 |
| Filed:
|
November 27, 1996 |
| Current U.S. Class: |
528/103; 257/E21.505; 257/E23.04; 528/117; 549/554 |
| Intern'l Class: |
C08L 063/00; C07D 303/28 |
| Field of Search: |
528/103,117
549/554
|
References Cited
U.S. Patent Documents
| 3522210 | Jul., 1970 | Sellers et al. | 260/47.
|
| 4110354 | Aug., 1978 | Bertram et al. | 260/348.
|
| 5536855 | Jul., 1996 | Schultz et al. | 549/539.
|
| Foreign Patent Documents |
| 0 028 024 A1 | May., 1981 | EP | .
|
| 0 028 024 | Jun., 1981 | EP | .
|
| 0 205 402 A2 | Dec., 1986 | EP | .
|
| 0 205 402 | Dec., 1986 | EP | .
|
| 0 272 442 A2 | Jun., 1988 | EP | .
|
| 0 397 317 A1 | Nov., 1990 | EP | .
|
| 0 397 317 | Nov., 1990 | EP | .
|
| 0 459 591 | Apr., 1991 | EP | .
|
| 0 459 591 A2 | Dec., 1991 | EP | .
|
| 0 747 370 A1 | Nov., 1996 | EP | .
|
| 0 747 371 A1 | Nov., 1996 | EP | .
|
| 1017612 | Jan., 1963 | GB.
| |
| WO 95/23794 | Aug., 1995 | WO | .
|
Primary Examiner: Wilson; Donald R.
Attorney, Agent or Firm: Gennaro; Jane E.
Parent Case Text
This application is a division of application Ser. No. 08/656,597, filed
May 31, 1996, now U.S. Pat. No. 5,646,315, which is a continuation-in-part
of Ser. No. 08/482,541, filed Jun. 7, 1995, abandoned.
Claims
We claim:
1. A flexible epoxy compound, characterized in that it has a total chlorine
content of less than 0.1% by weight, having the structural formula:
##STR21##
in which: R' is H, C.sub.1-18 alkyl, C.sub.1-5 alkoxy or aryl or
alkylaryl, C.sub.1-5 perfluoroalkyl, or C.sub.1-5 acyl; and
n is an integer 1-3; and
p is an integer 6-16.
2. A flexible epoxy compound according to claim 1 having the structure:
##STR22##
3. An adhesive for use in microelectronics applications containing a
flexible epoxy compound according to claim 1.
Description
FIELD OF THE INVENTION
This invention is directed to flexible epoxy compounds for use in
microelectronics adhesives that can be rapidly cured and that have low or
no ionic contamination, and to processes for their syntheses.
BACKGROUND OF THE INVENTION
One step in the manufacture of semiconductor integrated circuits is the
bonding of a silicon chip or die with an adhesive to a copper frame from
which extend metal conductor leads. The bonded die and lead frame assembly
is encapsulated within a polymeric sealant and connected to external
circuitry by way of the metal conductor leads that extend through the
encapsulation.
Epoxy compounds are preferred as the die attach or encapsulating adhesive
due to their superior adhesive strength. The epoxies conventionally used
are the aromatic epoxies, due to their strength, but these are inherently
rigid and brittle. During the manufacturing process the adhesives and
substrates are subjected to repeated thermal cycling. If the adhesives and
substrates have widely disparate coefficients of thermal expansion, the
stress of thermal cycling can lead to adhesive failure, substrate warpage,
or fracture of the die. Thus, a crucial requirement for an adhesive
destined for microelectronics use is that it be strong and flexible to
absorb the stress of thermal cycling.
A second crucial criterion is that the adhesives be capable of rapidly
curing to meet the speeds of assembly line processing. The fast cure times
required, typically 30-60 seconds at about 200.degree. C., are known as
snap-cure. This combination of criteria, adhesive strength, flexibility,
and ability to be snap-cured, is difficult to attain in one adhesive.
It is also crucial that the epoxy formulations be free of ionic
contamination, particularly sodium and chloride ions, and free of bonded
chlorine. These contaminants can cause corrosion of the metal leads in
semiconductor devices and the ultimate failure of the devices.
To add flexibility, epoxies can be co-reacted with an aliphatic
flexibilizer; this, however, reduces adhesive strength because the level
of aromatic moieties is lowered. The addition of a flexibilizer also
reduces the ability of the adhesive to be snap-cured because the
flexibilizer has a high molecular weight per epoxy. Moreover, when the
aliphatic and aromatic epoxies are cured, they may not co-react due to a
difference in reactivity rates; low molecular weight compounds may
volatilize out before cure, and high molecular weight compounds may not
completely cure. This combination of factors, sometimes even a problem for
slow cure formulations, is fatal for achieving snap-cure.
As a possible solution to some of these problems, it is known to combine
aromatic moieties with aliphatic moieties in the backbone of the same
polymer, but the currently available polymers of this type have a high
ratio of aromatic to aliphatic moieties, which results in a loss of
flexibility. Additionally, the method of preparation of these materials
results in high chlorine contamination, which is deleterious in
microelectronics applications. The polymers also have high viscosities,
which requires the addition of a solvent as a diluent. During snap-cure,
the curing is sometimes faster than complete solvent volatilization, which
leads to voids in the cured adhesive, and potential failure of the
microelectronics chip or device. Thus, low viscosity materials, which
eliminate the need for solvent, are preferred.
The problems associated with providing strength to flexible compounds, and
with snap-cure, are amplified as the microelectronics industry moves
toward larger and larger die sizes. This creates a continuing need for
improved flexible epoxies, which have strength and purity, and that can be
formulated into snap-cured die attach adhesives.
SUMMARY OF THE INVENTION
This invention relates to flexible epoxy resins that can be used to
formulate snap-curable die attach adhesives, encapsulants, and coatings.
The epoxies are prepared by a synthetic route that avoids the use of
epichlorohydrin resulting in low levels of ionic contaminated sources
(including sodium ion, chloride ion, and organic bound chlorine), less
than about 0.1% by weight, compared to prior art compositions, which
contain impurities at higher levels, especially when the compositions are
not subjected to stringent purification methods that significantly reduce
yields. Moreover, the synthetic routes permit the preparation of discreet
chemical structures, rather than mixes of various resins.
The epoxy resins are liquids at room temperature, have Tg values when cured
of less than or equal to 150.degree. C., and have neat resin viscosities
of 25,000 cps or less at 25.degree. C. Thus, no solvent is required for
formulating these epoxies into highly filled adhesives (for example, with
thermal or conductive fillers).
The modulus of elasticity, percent strain and glass transition temperature
(T.sub.g) values for these epoxies indicate high flexibility. The modulus
typically is less than or equal to 2.0 MPa. The percent strain typically
is greater than or equal to 5%. The cured T.sub.g values for these epoxies
will be 150.degree. C. or less, and for some compounds will be 100.degree.
C. or less.
In another embodiment, this invention pertains to adhesives made with these
epoxies, for use in microelectronics applications. The flexibility of
these compounds is retained when the epoxies are formulated with
conductive material, such as silver flakes, and used to bond silicon chips
to a metal lead frame substrate. The cured epoxies demonstrate a high
radius of curvature, greater than or equal to 350 mm. (The higher the
radius of curvature, the less chip warpage, which in turn implies a
flexible adhesive.)
The cured epoxies also demonstrate good adhesive strength, measured as die
shear strength. Die shear is a measure of the force required to remove a
bonded chip from the metal substrate, which for these compounds is greater
than or equal to 13 MPa.
Moreover, the adhesives exhibit no loss of flexibility or strength.
Preferably the formulation will comprise 20-80 parts by weight of the
flexible epoxy and 80-20 parts of an aromatic O-glycidyl ether (to a total
of 100 parts), a curing agent, a conductive filler, and optionally 20-50
parts of a phenolic hardener per hundred parts of epoxy resin (pphr).
DETAILED DESCRIPTION OF THE INVENTION
The structural composition of the epoxy resins comprises an oligomeric
backbone of alkylene or alkyleneoxy repeat units, of moderate length to
provide flexibility, but still maintain sufficient crosslink density to
yield useful adhesive properties. The flexible backbone is terminated on
one or both ends with an aromatic moiety bearing one or more epoxy
functionalities. Thus, the compounds have no more than two aromatic
moieties, which exist only at the termini. Restricting the number and the
location of aromatic moieties in the polymer backbone maintains the
strength imparted to the cured material by the aromatic moieties and
maximizes the flexibility imparted to the cured material by the aliphatic
moieties.
The compounds have no more than two hydroxyl groups per molecule;
restricting the number of hydroxyl groups to two or less promotes low
viscosity.
The epoxies have one of the following general structural formulas:
##STR1##
where flexible chain is CH2CH2CH2CH2O when q=1-6 or where flexible chain
is CH2CH2O when q=3-10
##STR2##
where flexible chain is CH2CH2CH2CH2O when r=1-10 or where flexible chain
is CH2CH2O when r=3-10
or where flexible chain is CH2 when r=10-20
in which R' is H, C.sub.1-18 alkyl, C.sub.1-5 alkoxy or aryl or alkylaryl,
C.sub.1-5 perfluoroalkyl, or C.sub.1-5 acyl; n is an integer 1-3; the
flexible chain is as described for each structure; and p, q, and r am
integers as described for each structure.
As used here, alkyl refers to a hydrocarbon group derived from an alkane
and have the generic formula C.sub.n H.sub.2n+1 ; alkoxy refers to an
alkyl group also containing oxygen; aryl refers to a group having the ring
structure characteristic of benzene; alkylaryl refers to a group
containing both alkyl and aryl structures; perfluoroalkyl refers to an
alkyl group in which one or more of the hydrogens are substituted with
fluorine; and acyl refers to an organic acid group in which the OH of the
carboxyl group is replaced by some other substituent, in this case, the
phenyl ring to which it is attached.
The flexibilized epoxy resins may be prepared by any suitable synthetic
method that leaves the product free of ionic contamination. The preferred
synthetic routes are those that avoid the use of epichlorohydrin, and
comprise the steps of (a) preparing an olefinic precursor by attaching an
alkylene or alkyleneoxy chain to a phenoxy compound bearing olefinic
groups, and (b) oxidizing the olefinic groups to the corresponding
epoxides.
In step (a), the flexible alkylene or alkyleneoxy chain is end terminated
with a functional group that is activated toward nucleophilic
substitution. Preferably the functional group is an epoxy, chloro,
sulfonate, or toluenesulfonate. The flexible chain is derivatized by
nucleophilic addition of a phenolate compound displacing the chloride,
sulfonate, or alkoxide (from epoxy ring opening) functional group. The
phenolate is prepared from the reaction of sodium hydroxide or other
alkali metal hydroxide with a phenol compound having only one hydroxyl
group, which assures that the nucleophilic substitution can occur only
once and discrete chemical structures result (that is, not mixtures of
products). The phenol compound is also substituted with an olefin moiety,
preferably an allyl group, which is inert to nucleophilic substitution,
and serves as the site for the final epoxide product. In step (b), the
olefin on the phenol group is converted to the oxirane (epoxide).
Preparation of the Olefinic Precursors
The olefinic precursors to the epoxy compound preferably may be prepared by
one of the following generally described methods. More specific detail for
each method is provided in the examples.
I. A diglycidyl compound of an alkylene or alkyleneoxy chain, of
appropriate length to conform in the final product to one of the above
described general structures, may be reacted with a phenolic compound
bearing one or more olefinic groups in the presence of a catalyst
effective for catalyzing condensation reactions between phenolic compounds
and oxiranes.
Preferred diglycidyl compounds are 1,4-butanediol diglycidyl ether;
1,12-dodecanediol diglycidyl ether; polyethylene glycol digylcidyl ether,
poly(tetra-methylene glycol) diglycidyl ether, and polypropylene glycol
diglycidyl ether in which the polyglycol portion of the molecule has a
molecular weight less than or equal to 2000 daltons;
1,2,9,10-diepoxydecane, 1,2,13,14-diepoxytetradecane, and similar
compounds; and the diglycidyl ether of poly(caprolactone)diol.
Preferred olefin functionalized phenolic compounds are 2-allylphenol;
2,6-diallylphenol; 2,4-diallylphenol; 4-allyl-2-methoxyphenol; and other
similar phenolic compounds. The phenolic compound can be used in excess to
effect a high degree of reaction with the diglycidyl compound and the
excess can be removed from the product by distillation. The phenolic
compound may also be substituted with groups that do not interfere with
either of the two steps involved in the formation of the product.
Preferred catalysts are quaternary ammonium or phosphonium salts of
chloride, bromide, hydrogen sulfate, acetate, and other known catalysts,
such as, triphenyl-phosphine. More preferred catalysts are the quaternary
ammonium salts with lower alkyl constituents, particularly tetramethyl
ammonium chloride.
II. An alkylene or alkyleneoxy chain, of appropriate length to conform in
the final product to one of the above described general structures, and
bearing a leaving group, such as, chloride, bromide, iodide, or tosylate,
at the termini of the chain, may be reacted with an alkali or alkaline
earth metal salt of a phenoxide compound bearing olefinic groups.
The alkylene or alkyleneoxy compound bearing the leaving group generally is
derived from the corresponding alcohol by conversion of the hydroxyl group
into a leaving group using known reactions. For example, an alkylene or
alkyleneoxy chloride can be generated from the alcohol by reaction with
SOCl.sub.2. The tosylate can be generated from the alcohol by reaction
with tosyl chloride in the presence of base. Examples of suitable alcohol
terminated alkylene or alkyleneoxy chains include 1,4-butanediol;
1,12-dodecanediol; polycaprolactone diol; polyethylene glycol; and
1,10-decanediol.
The olefin functionalized phenoxide compound is generated from the
corresponding phenolic compound by the action of an alkali or alkaline
earth metal hydroxide in the presence of an azeotroping solvent. These
reactions are well known in the art. Suitable azeotroping solvents include
toluene, xylene, p-cymene, and similar solvents.
Epoxidation of the Olefinic End Groups
After the precursors are formed, the olefin in oxidized by any method
effective to provide the corresponding epoxy. Various methods are
described in the literature for effectively carrying out the oxidation
process, and include the use as an oxidant of organic peracids, such as
peracetic acid or trifluroperacetic acid; peroxyimidic acids; dioxirane
compounds like dimethyldioxirane; inorganic peracids, such as
peroxytungstate systems; metal-catalyzed systems, such as
methyltrioxorhenium and hydrogen peroxide, titanium catalyzed
t-butyl-hydroperoxide, and manganese-salen catalyzed oxidations.
The preferred epoxidation methods use inorganic peracids, peracetic acid,
and peroxyimidic acids as the primary oxidant. More specific detail for
each method is provided in the examples.
The preferred inorganic peracids are based on tungstic acid,
phosphotungstic acid, and similar compounds, using hydrogen peroxide as
the secondary oxidant. The oxidation process is enhanced by employing a
two phase system and a quaternary ammonium or phosphonium salt as the
phase transfer catalyst.
Peracetic acid in acetic acid may also be used successfully as the oxidant.
Although many solvents are suitable for this oxidation, the rate is faster
in chlorinated solvents, such as, dichloroethane. In this process, as the
reaction is pushed to complete conversion, the accumulating acetic acid
begins to attack the oxirane. The result is a product with a lower oxirane
content (higher WPE), and higher viscosity than if the oxidizing agent is
an inorganic peracid in the presence of a quaternary onium salt.
The use of peroxyimidic acid as the oxidant provides the advantage that the
byproduct is acetamide, which is innocuous to the oxirane. A preferred
peroxyimidic acid is derived from acetonitrile and hydrogen peroxide. The
optimum conditions for the peroxyimidic acid oxidation include the use of
KHCO.sub.3, reaction temperature near 40.degree. C., and optionally, the
use of an aqueous sponge, such as, 2,2-dimethoxypropane.
Formulation of Snap-Cure Adhesives
The flexible epoxy compositions of this invention are useful as oven cured
adhesives; however, their utility can be expanded if the snap-cure
property is available. The epoxies can be formulated into adhesives that
exhibit good adhesive strength, flexibility, and ability to be snap-cured.
The preferred formulations will contain 20-80 parts by weight of the
flexible epoxy and 80-20 parts by weight of an aromatic O-glycidyl ether
resin (to total 100 parts), a curing catalyst, and optionally one or more
fillers. In general the curing catalyst is an imidazole catalyst, present
in an amount of about 5-10 parts per hundred parts resin, and the filler
is thermally or electrically conductive, such as silver flakes, although
other electrically or thermally conductive fillers or silica may be used.
The filler is preferably present in an amount of about 25% by volume of
the adhesive formulaton.
The aromatic O-glycidyl ether (aromatic epoxy) will be substituted with one
or more aromatic rings, and optionally with one or more C.sub.1 -C.sub.3
groups. The aromatic epoxies will have an epoxy equivalent weight (WPE,
weight per epoxy) of 200 or less, and will be derived form the
corresponding phenolic hardeners. Exemplary aromatic epoxies are
bisphenol-Fdiglycidyl ether, bisphenol-A diglycidyl ether, resorcinol
diglycidyl ether, and the epoxy phenol novolaks.
The adhesive formulation may further contain 20-50 parts per hundred parts
of resin of a phenolic hardener having one or more hydroxyl groups per
aromatic ring (from which are derived the aromatic epoxies described
above). Exemplary phenolic hardeners are the commercially available phenol
novolak resins, bisphenol-F, bisphenol-A and resorcinol. The addition of
the phenolic resin serves to improve the adhesion strength of the
formulation after exposure to moisture and thermal shock at 250.degree.
C., and to prevent bleeding of the flexible epoxy out of the formulation.
In general any effective curing catalyst in an effective catalytic amount
can be used. The preferred curing catalyst is an imidazole catalyst, and
typically is present in an amount of about 5 parts per hundred parts
resin. Preferable imidazole catalysts are imidazole, 2-ethylimidazole,
2-ethyl-4-methylimidazole, 2-phenylimididazole,
2-phenyl-4-methylimidazole, and 2-undecylimidazole.
The filler may be any of the thermally or electrically conductive materials
known to be suitable for microelectronics applications, used in any
effective amount. For many applications, the preferred filler is silver
flakes, although other electrically or thermally conductive fillers or
silica may be used, and is preferably present in an amount of about 25% by
volume of the adhesive formulation.
EXAMPLES
The following examples illustrate the preparation of representative
olefinic precursors (ADDUCTS) by the two preferred synthetic routes, and
the epoxidation of the olefins by the three preferred epoxidation methods
to give the corresponding epoxy compounds (EPOXIES). The evaluation test
methods are given and the performance results tabulated.
The examples also disclose snap-curable die attach adhesive formulations
that contain the flexible epoxies and the O-glycidyl ether aromatic
epoxies, and formulations containing the phenolic resin hardeners.
Test Methods
The following test methods were used to evaluate the sample epoxy resins.
The epoxy equivalent weight (WPE, weight per epoxy) for each of the epoxy
products was determined by dissolving the compound in glacial acetic acid
followed by titration with standardized H Br/acetic acid (.about.0.1N) to
the violet/green transition of crystal violet indicator.
The viscosity of each of the samples was determined using a Brookfield
cone-n-plate viscometer at multiple spindle speeds at 25.degree. C. and is
reported as an average of three readings.
The amount of hydrolyzable chlorine was determined by the following method:
Weigh accurately 1-5 g (to the nearest 0.1 mg) of resin into a clean, 125
ml Erlenmeyer flask with a Teflon.RTM. coated magnetic stir bar. Add 40 ml
of 0.1N KOH dissolved in methanol. Place flask fitted with reflux
condensor into a water bath heated by magnetic stirrer/hot plate unit.
Reflux the stirred solution for exactly 15 minutes, then remove the flask,
and allow it to cool to room temperature. Transfer the liquid into a clean
250 ml beaker. Rinse the sample flask three times with 50 ml portions of
methanol, transferring the liquid into the beaker. Add 10 ml of glacial
acetic acid and titrate the chloride ion potentiometrically to the end
point with 0.005N AgNO.sub.3 solution. Calculate the hydrolyzable chloride
content in ppm as follows:
##EQU1##
The amount of total chlorine was determined by the following method:
Accurately weigh 1-5 g (to the nearest 0.1 mg) of resin into a clean
Erlenmeyer flask with a magnetic stir bar. Add 30 ml of dioxane and 15 ml
of 3N KOH solution in ethanol. Place flask fitted with a reflux condensor
into a 100.degree. C. water bath heated on a stirrer/hot plate unit. Allow
sample solution to reflux for 30 minutes. Remove flask and allow it to
cool to room temperature. Transfer the liquid into a clean 400 ml beaker.
Rinse the sample flask three times with 50 ml portions of methanol,
transferring the liquid into the beaker. Add 100 ml of glacial acetic
acid. Titrate potentiometrically with 0.005N AgNO.sub.3 solution to the
end point using a silver electrode with a KNO.sub.3 salt bridge. Calculate
the chloride content in ppm as described above.
Example I
Syntheses of the Olefinic Precursors
##STR3##
The reaction apparatus was a 500 ml round bottom flask equipped with a
mechanical stirrer, thermometer, and condenser. The reagents, 2-allyl
phenol (159 gms, 1.19 moles), 1,4-butanediol diglycidyl ether (120 gms,
0.59 moles), and 1% tetramethylammonium chloride catalyst (2.5 gms), were
added to the reaction apparatus and stirred at 50.degree. C. for
approximately an hour. The temperature was raised to 70.degree. C. and the
reagents stirred at that temperature for an additional hour; then the
temperature was raised to 120.degree. C. and the reagents held at that
temperature while progress of the reaction was monitored by gas
chromotograpy (GC). Completion was determined to occur after four hours,
at which time the reaction was cooled and the work-up initiated.
The reaction mixture was transferred to a single neck flask and placed on a
kugelrohr apparatus (120.degree. C./0.1 mmHg) to strip off any residual
starting materials (particularly, 2-allyl phenol).
The adduct was transferred to a 1000 ml separatory funnel and dissolved in
300 ml of toluene. This organic phase was washed with 5% sodium carbonate
(3.times.200 mL), 10% sodium sulfate (3.times.200 mL), and then passed
through a neutral alumina pad (.about.200 mL) rinsed with toluene.
The toluene was stripped off by rotory evaporator at 60.degree. C./25 mm Hg
vacuum, and then placed on a kugelrohr at 70.degree. C./0.1 mm Hg to
remove any residual solvent.
The adduct was obtained in 90% yield, and was characterized by IR and
.sup.1 H NMR spectroscopy. It had a viscosity ranging from 650 to 870 cps
at 25.degree. C.
##STR4##
Adduct B was prepared in a manner similar to that used for Adduct A, except
that 2,6-diallyl phenol (310 gms, 1.78 moles) was reacted with
1,4-butanediol diglycidyl ether (180 gms, 0.89 moles) in the presence of
4.5 gm tetramethylammonium chloride. The temperature of the reaction
mixture was gradually raised to 180.degree. C. and held there for one hour
until reaction was complete as indicated by GC. The product was isolated
as described for Adduct A in 81% yield and was characterized by IR and
.sup.1 HNMR spectroscopy. It had a viscosity of 983 cps at 25.degree. C.
##STR5##
Adduct C was prepared in a manner similar to that used for Adduct A, except
that 2-allylphenol (8.44 gm, 0.063 moles) was reacted with
1,2,13,14-diepoxy tetradecane (6.48 gm, 0.0286 moles; prepared by the
oxidation of 1,13-tetradecadiene) in the presence of 0.1 gm
tetramethylammonium chloride. The reaction mixture was heated to
185.degree. C. and held at that temperature for 3.3 hour until reaction
was complete as indicated by GC. The product was isolated as described for
Adduct A in 81% yield and was characterized by IR and .sup.1 HNMR
spectroscopy.
##STR6##
The reaction apparatus was a one liter multi-neck flask equipped with
mechanical stirrer, thermometer, Dean-Stark trap, and condenser. Solid
sodium hydroxide pellets (19.95 gm, 0.5 moles) and 83 mL xylene under a
gentle nitrogen purge were added to the apparatus and heated with a
heating mantle to 70.degree. C. At that temperature 2-allyl phenol (66.9
gm, 0.5 moles) dissolved in 200 mL xylene was introduced through a
slow-addition funnel, while the system was brought to reflux. The water
formed as a by-product was driven off via azeotropic distillation to the
Dean-Stark trap, followed by distillation of additional xylene (225 mL)
through the Dean-Stark trap to remove any trapped water.
Then 125 mL p-cymene, 25 mL dimethylformamide, and 0.1 gm Kl were added and
the entire contents cooled to 100.degree. C. The dichloride of
poly(tetrahydrofuran)-250 (45 gm, 0.17 mole; prepared by the action of
thionyl chloride on poly(tetrahydrofuran)-250, which is available from
BASF Corp.) dissolved in 50 mL p-cymene was introduced into the flask
through a slow-addition funnel. The reaction temperature was raised to
150.degree. C. and held for 24 hours. Upon cooling, the by-product salts
(sodium chloride) were removed by filtration, and the filtrate was washed
with 5% sodium hydroxide (2.times.50 mL), and then 10% sodium sulfate (150
mL each) until the pH became neutral (pH=7 on test strip). The solvent was
removed first on a rotory evaporator, then on a Kugelrohr apparatus
(130.degree. C./0.2 mm Hg). The product was collected in 95% yield and was
analyzed by IR and .sup.1 HNMR spectroscopy.
##STR7##
The reaction apparatus was a five liter multi-neck flask equipped with
mechanical stirrer, thermometer, Dean-Stark trap, and condenser. Solid
sodium hydroxide pellets (79.2 gm, 1.98 moles) and one liter xylene were
added to the reaction apparatus under a gentle nitrogen purge, and heated
with a heating mantle to 135.degree. C. (reflux). At that temperature
2-allyl phenol (267 gm, 1.98 moles) was introduced through a slow-addition
funnel, while the system was maintained at reflux. The water formed as a
by-product was driven off via azeotropic distillation to the Dean-Stark
trap, followed by distillation of 100 mL additional xylene through the
Dean-Stark trap to remove any trapped water.
The internal temperature was lowered to 90.degree. C., and 100 mL
dimethylformamide and tetraethylene glycol ditosylate (400 gm, 0.795
moles; prepared by the action of p-toluenesulfonyl chloride on
tetraethylene glycol in the presence of pyridine) dissolved in 300 mL
xylene were introduced into the flask through a slow-addition funnel. The
reaction temperature was raised to 120.degree. C. and held there for two
hours. The reaction was cooled and the by-product salts (sodium tosylate)
were removed by filtration. The filtrate was washed with 5% sodium
hydroxide (2.times.600 mL), then washed with 10% sodium sulfate
(3.times.300 mL) until the pH became neutral (pH=7 on test strip).
The solvent was removed first on a rotory evaporator, then on a Kugelrohr
apparatus (80.degree. C./0.2 mm Hg). The product was collected in 95%
yield and analyzed by IR and .sup.1 HNMR spectroscopy.
Example II
Syntheses of the Epoxy Compounds
##STR8##
Adduct A was oxidized to give Epoxy A by two methods: with peracetic acid
and with peroxyimidic acid.
1. Oxidation with Peracetic Acid
The reaction apparatus was a 500 ml flask equipped with mechanical stirrer,
thermometer and slow-add funnel. Adduct A (75 gms, 0.16 moles, 0.32 moles
of olefinic functionality), 1,2-dichloroethane (150 mL) and sodium acetate
(4.7 gms) was added to the reaction apparatus and brought to 23.degree. C.
using a water bath. Introduction of peracetic acid (94.7 gms) (35% in
acetic acid) was begun, and the reaction exothermed to 29.degree. C. After
addition was completed, the water bath was warmed to 30.degree. C. The
internal temperature rose to 38.degree. C. After 10 minutes the
temperature dropped to 35.degree. C. and was held steady at 35.degree. C.
for 5 hours. The reaction was monitored for completion by IR spectroscopy
for the disappearance or minimization of the 1635 cm.sup.-1 absorption.
At completion, 300 mL of distilled water was added to the reaction mixture.
The phases were separated, and the organic phase washed with 300 mL 5%
sodium bicarbonate solution, then with 200 mL of 5% sodium sulfite
solution. Toluene (300 mL) was added to break the emulsion. The organic
layer was then washed with 200 mL of 10% sodium sulfate, and 200 mL of
distilled water.
The organic was separated off, dried over magnesium sulfate and suction
filtered. The solvent was stripped off first on the rotory evaporator
(50.degree. C./20 mmHg) and then on a Kugelrohr apparatus (60.degree.
C./0.1 mmHg). The final product was isolated as an oil, and characterized
by a viscosity of 7280 cps at 25.degree. C., an epoxy equivalent
weight=269 (theoretical value=251), and by IR and .sup.1 H NMR
spectroscopy. Total chlorine=717 ppm. In a second preparation of this
material the EEW=274, viscosity=6550 cps, hydrolyzable chlorine=519 ppm,
and total chlorine=719 ppm.
2. Oxidation with Acetonitrile/Hydrogen Peroxide (Peroxyimidic acid)
The reaction apparatus was a 250 ml round bottom flask equipped with
mechanical stirrer, thermometer, and slow-add funnel. Adduct A (30.0 gm,
0.0638 moles, 0.128 moles olefin functionality), methanol (74 mL),
acetonitrile (11.54 gm, 0.2815 moles), and KHCO.sub.3 (7.12 gm, 0.7111
mole) were added to the apparatus and heated to 40.degree. C. At that
temperature 30% H.sub.2 O.sub.2 (10.0 gm, 0.0882 mole) was added dropwise.
The reaction mixture was maintained at 40.degree. C. for seven hours, at
which point, additional 30% H.sub.2 O.sub.2 (13.8 gm, 0.1218 mole) was
added dropwise. The reaction was maintained at 40.degree. C. for a further
20 hours.
Then 2,2-dimethoxypropane (48 gm, 0.462 mole, or 0.5 equivalent based on
H.sub.2 O charged from 30% hydrogen peroxide) was added dropwise. The
reaction was heated at 40.degree. C. for an additional 24.5 hours,
corresponding to the point at which the IR trace of a reaction sample
indicated minimal 1637 cm.sup.-1 absorbance.
The peroxide was neutralized by the addition of 10% Na.sub.2 SO.sub.3 as
determined by Kl-starch indicator test strip. The methanol and
acetonitrile were removed in vacuo on the rotory evaporator (40.degree.
C./25 mm HG). The residue was taken up in 150 mL toluene and washed with
distilled water (150 mL). Emulsification was alleviated by the addition of
10% Na.sub.2 SO.sub.4. The organic layer was separated and the solvent
removed first on a rotory evaporator (80.degree. C./25 mmHg), then on a
Kugelrohr apparatus (80.degree. C./0.1 mmHg).
The product was isolated at 74% yield (23.7 gm), characterized by an epoxy
equivalent weight=315 (theoretical value=251), viscosity=6270 cps at
25.degree. C., IR, and .sup.1 H NMR spectroscopy. Hydrolyzable
chlorine=455, and total chlorine=564 ppm.
##STR9##
Adduct B was oxidized to give Epoxy B by two methods: with peracetic acid
and with peroxyimidic acid.
1. Oxidation with Peracetic Acid
Adduct B was oxidized using 35% peracetic acid as described for the
synthesis of Epoxy A. Adduct B (110 gm, 0.2 mole) was treated with
peracetic acid (210 mL of 35% solution, 1.09 moles) in 220 mL
1,2-dichloroethane at 35.degree. C. The product was isolated in 85% yield
and characterized by an epoxy equivalent weight=176 (theoretical
value=154), viscosity=24,300 cps at 25.degree. C., and by IR and .sup.1 H
NMR spectroscopy. Hydrolyzable chlorine=467 ppm, and total chlorine=620
ppm.
2. Oxidation with Acetonitrile/Hydrogen Peroxide (Peroxyimidic acid)
Adduct B was oxidized using acetonitrile and hydrogen peroxide as described
for the synthesis of Epoxy A. The acetonitrile and hydrogen peroxide
generated in situ the peroxyimidic acid. Adduct B (150 gm, 0.27 mole) was
treated with acetonitrile (97 gm), potassium bicarbonate (60.5 gm), 30%
hydrogen peroxide (200 gm), 2,2-dimethoxypropane (812.5 gm), and methanol
(375 mL). The reaction was carried out at 40.degree. C. until the IR
absorption at 163 cm.sup.-1 no longer changed significantly. The product
was isolated in 74% yield, and was characterized by an epoxy equivalent
weight=229 (theoretical value=154), viscosity=20,900 cps at 25.degree. C.,
and by IR and .sup.1 H NMR spectroscopy. Hydrolyzable chlorine=624 ppm,
and total chlorine=839 ppm.
##STR10##
Adduct C was oxidized to give Epoxy C using peroxytungstate catalysis.
Oxidation with Peroxytungstate
Adduct C (17 gm, 0.035 mole) was dissolved in 60 mL dichloroethane to which
was added 3.8 gm tetrabutylammonium hydrogen sulfate, and this was stirred
vigorously at room temperature. To this rapidly stirred solution was added
an aqueous mixture comprising sodium tungstate dihydrate (6.3 gm) in 10 mL
distilled water with a sufficient amount of 85% phosphoric acid to adjust
the pH to 6.5. Then 30% hydrogen peroxide (15.7 gm, 0.138 mole) was added
at room temperature and the combined contents were stirred for 96 hours.
At that point, the 1637 cm.sup.-1 absorption in the IR spectrum was
minimized.
The product was isolated by separating the phases and washing the organic
layer with 5% sodium sulfite (2.times.100 mL) until the peroxide was
completely quenched. The solvent was removed on a rotory evaporator at a
temperature less than or equal to 40.degree. C. and the residue dissolved
in 100 mL toluene. The toluene layer was washed with 150 mL 5% sodium
carbonate, with water (2.times.150 mL), then with 10% sodium sulfate
(2.times.150 mL). The toluene was removed from the organic layer first on
a rotory evaporator, then on a Kugelrohr apparatus under high vacuum
(80.degree. C./0.1 mm Hg). The product was an oil isolated at 93% yield,
and was characterized by an epoxy equivalent weight=289 (theoretical
value=263), viscosity=9100 cps at 25.degree. C., and by IR and .sup.1 H
NMR spectroscopy. Hydrolyzable chlorine=883 ppm, and total chlorine=959
ppm.
##STR11##
Adduct D was oxidized to Epoxy D by two methods: with peracetic acid and
with peroxytungstate catalysis.
1. Oxidation with Peracetic Acid
Adduct D (50 gm, 0.107 mole) was oxidized as described in the synthesis of
Epoxy A using 35% peracetic acid at 35.degree. C. in 1,2-dichloroethane.
The product was isolated as an oil at 60% yield, and was characterized by
an epoxy equivalent weight=274 (theoretical value=249), viscosity=417 cps
at 25.degree. C., and by IR and .sup.1 H NMR spectroscopy. Hydrolyzable
chlorine=35 ppm, and total chlorine=103 ppm.
2. Oxidation with Peroxytungstate Catalysis
Adduct D (67.8 gm, 0.14 mole) was oxidized as described in the synthesis of
Epoxy C using sodium tungstate dihydrate and hydrogen peroxide. The
product was isolated as an oil in 83% yield, and was characterized by an
epoxy equivalent weight=252 (theoretical value=249), viscosity=410 cps at
25.degree. C., and by IR and .sup.1 H NMR spectroscopy. Hydrolyzable
chlorine=53 ppm, and total chlorine=100 ppm.
##STR12##
Adduct E was oxidized to Epoxy E by two methods: with peracetic acid and
with peroxytungstate catalysis.
1. Oxidation with Peracetic Acid
Adduct E (30 gm, 0.07 mole) was oxidized as described in the synthesis of
Epoxy A using 35% peracetic acid at 35.degree. C. in 1,2-dichloroethane.
The product was isolated as an oil in 91% yield and was characterized by
an epoxy equivalent weight=258 (theoretical value=229), viscosity=397 cps
at 25.degree. C., and by IR and .sup.1 H NMR spectroscopy. In a second
preparation of this material the EEW=259, viscosity=354 cps, hydrolyzable
chlorine=below detection limits, and total chlorine=785 ppm.
2. Oxidation with Peroxytungstate Catalysis
Adduct E (24 gm, 0.056 mole) was oxidized as described in the synthesis of
Epoxy C using sodium tungstate dihydrate and hydrogen peroxide. The
product was isolated as an oil in 94% yield, and was characterized by an
epoxy equivalent weight=241 (theoretical value=229), viscosity=347 cps at
25.degree. C., and by IR and .sup.1 H NMR spectroscopy. Hydrolyzable
chlorine=58 ppm, and total chlorine=578 ppm.
##STR13##
As a comparison to the inventive examples, Epoxy F was prepared according
to the methods described in U.S. Pat. No. 3,522,210, issued to Sellers,
examples 1A and 15.
Preparation of bisphenol compound (example 1): To a one liter flask with
mechanical stirrer, thermometer, slow addition funnel, condenser, and
nitrogen blanket was charged bisphenol A (228.29 g, 1.0 mole), 150 g
dimethyl sulfoxide (DMSO), and 150 g toluene. After the solid dissolved a
solution of 50% aqueous NaOH (80 g, 1.0 mole) was added over 20 minutes.
The temperature rose to about 50.degree. C. and heat was applied to raise
the temperature to 100.degree.-110.degree. C. Reflux was achieved at
101.degree. C. at which point 2-chloroethyl ether (71.5 g, 58.6 mL, 0.5
mole) was added through the slow addition funnel over 18 minutes. The
reaction temperature was held at reflux for 36.75 hours at which point the
pH still did not reach 7-8 but GC analysis indicated the reaction had
stalled with respect to consumption of bisphenol A. Toluene (150 mL) was
added and azeotropic distillation was initiated to drive off .about.60 mL
H.sub.2 O and another 40 mL of toluene was driven off additional to ensure
dryness. The organic solution was filtered, neutralized to pH=8 with conc.
HCl, and refiltered. The solution was placed in a single-neck round bottom
flask and stripped on a rotary evaporator to 60.degree. C./11 mmHg,
collecting 300 mL distillate. Further distillation was carried out at
atmospheric pressure (.about.760 mmHg) and 170.degree. C. pot temperature,
then establishing vacuum to .about.30 mmHg and raising the pot temperature
to 219.degree. C. (vapor temperature to 122.degree. C.). The flask was
cooled to room temperature.
Preparation of Diglycidyl Ether with Flexible Linkage (example 15): The
material was dissolved in 400 g ethanol and 240 mL epichlorohydrin in
order to transfer to a 2 liter multi neck flask for the second reaction.
This solution was heated to 60.degree. C. at which point the addition of
50% NaOH (1.25 mole) .about.70 mL was conducted as prescribed (.about.7 mL
over 25 min., .about.10 mL over 5 min., .about.46 mL over 35 min., and
.about.7 mL over 15 min.). The reaction mixture was transferred to a
2-liter single neck flask and ethanol, water, and epichlorohydrin were
removed on a rotary evaporator to 96.degree. C./11 mmHg. The residue was
dissolved in 600 mL methyl isobutyl ketone and 800 mL water, transferred
to a separatory funnel and the phases separated. The organic layer was
washed with 200 mL water, which resulted in an emulsion that was
alleviated by the addition of 100 mL saturated sodium sulfate. The pH of
the aqueous wash was .about.7 (neutral). The organic solution was placed
on the rotary evaporator to remove solvent to 95.degree. C./11 mmHg. The
material was further stripped on a Kugelrohr apparatus to 133.degree.
C./1.5 mmHg. The product, a dark amber material with a stringy taffy-like
consistency, was isolated in 89% yield, and was characterized as having an
epoxy equivalent weight as WPE=357.5, hydrolyzable chlorine=435 ppm, bound
chlorine=2103 ppm, and total chlorine=2538 ppm.
______________________________________
SUMMATION OF CHLORINE DATA:
Hydrolyzable
Total Chlorine
Bound Chlorine
EPOXY Chlorine (ppm)
(ppm) (ppm)
______________________________________
A-1 519 719 200
A-2 455 564 109
B-1 467 620 153
B-2 624 839 215
C 883 959 76
D-1 35 103 68
D-2 53 100 47
E-1 none 785 785
detected
E-2 58 578 520
F-Comparative
435 2538 2103
______________________________________
In comparing the inventive epoxy compounds with prior art compound F, it
will be noted that the hydrolyzable chlorine content is similar, and it is
further noted that there are methods described in the literature for
treatment of the epoxide containing molecule to reduce this value. (See,
for example, U.S. Pat. No. 4,785,061, which discusses at column 2
hydrolyzable chlorine, bound chlorine, and total chlorine.) Nevertheless,
the total chlorine value is critical when considering the prevention of
corrosion of sensitive microcircuitry conductive lines, and reported
methods lower but do not effectively remove this chlorine contaminant from
an epichlorohydrin based synthetic route.
Example III
Flexibility and Strength of Epoxies
The epoxies were tested for modulus, percent strain, radius of curvature as
a measure of their flexibility and die shear as a measure of their
adhesive strength.
The modulus is a coefficient of elasticity representing the ratio of stress
to strain as a material is deformed under dynamic load. These epoxies
demonstrated a low modulus, preferably less than or equal to 2.0 MPa. The
percent strain is a measure of the resistance to deformation, and these
compounds have a percent strain of elongation, greater than or equal to
5%, and notably in certain cases the test specimen yielded.
When formulated with silver flakes and used to bond silicon chips to a
metal lead frame substrate, the cured epoxies demonstrated a high radius
of curvature, greater than or equal to 350 mm, and good die shear
strength, greater than or equal to 13 MPa at room temperature.
The cured T.sub.g values for these epoxies will be 150.degree. C. or less,
and for some compounds will be 100.degree. C. or less.
Modulus
The following equipment was used to prepare epoxy plaques for the modulus
and percent strain flexural testing: 2 pyrex plates 10.times.103/16" from
F.J. Gray Co.; copper wire (23" per test); silcon tubing ID=3/32",
OD=5/32", wall=1/32" (.about.23" per test); aluminum spacers
(2".times.3/4".times.1/8"); binder clips size medium (5/8"); Release ALL
#50 from Airtech International, Inc.
To prepare the form for the plaques, two glass (PYREX) plates were cleaned
with toluene, then acetone. A thin coat of Release ALL #50 was applied to
one side of each plate with a clean rag and allowed to dry for 10 minutes
at room temperature. A second coat of Release ALL #50 was applied over the
first coat. The plates were dried in a oven at 125.degree. C. for 30
minutes. A 23" length of copper wire was inserted into a 23" length of
silicone tubing, and the wire/tube bent into a "U" shape form to fit
between the two glass plates as a reservoir for the liquid epoxy
formulation before cure. Aluminum spacers were positioned around the edges
of the coated side of one glass plate. The wire/tube form was then laid on
the coated side of the glass plate, interior to the spacers. The second
glass plate, coated side to the wire/tube and spacers, was laid over the
first glass plate so that the wire/tubing form and spacers were sandwiched
between the two plates. The two plates of glass were clamped together
using the binder clips.
A sample of 90.25 g of epoxy resin was poured into a 250 mL single neck
flask. The sample was degassed on a Kugelrohr at 70.degree.-75.degree. C.
and 0.2 mm Hg for .about.20 minutes, and cooled to 50.degree. C. Based on
100 parts by weight of the epoxy, (4.75 gm) 5 parts by weight of
2-ethyl-4-methylimidazole was added with mixing to the sample. The sample
was degassed on a Kugelrohr at 50.degree. C. and 2 mmHg for .about.20
minutes.
The epoxy sample was poured into the cavity formed by the "U" shaped
wire/tube and glass plate mold with care taken to avoid forming air
pockets. The poured epoxy sample was cured in a programmable oven in an
upright position according to the following program: 50.degree. C. for 30
minutes; 50.degree. C. to 125.degree. C. @ 1.degree. C./min; hold at
125.degree. C. for 1 hour; 125.degree. C. to 175.degree. C. @10.degree.
C./min; hold at 175.degree. C. for 1 hour; cool to 60.degree. C. over 90
min.
The resulting epoxy plaques, after release from the mold, were cut into
3".times.0.5" pieces with a water-cooled diamond blade saw. The 0.5 cut
was cut with a slight excess so that the plaques could then be milled to
exactly 0.5".
Flexural testing was carried out on the plaques using an Instron 4507 at
room temperature in the three point bend mode following ASTM method
D790-84a. Data from these experiments is reported in the following Table
as Young's Modulus in MPa units, % strain, and T.sub.g (peak in Tan delta)
in .degree.C. units.
Die Shear
Each epoxy (to which was added 5 parts 2-ethyl-4-methyl imidazole as curing
catalyst to 100 parts by weight epoxy) was mixed with silver flake at 25%
volume until the silver was fully wetted out. The mixture was degassed
under vacuum at room temperature for 15-20 minutes, and then allowed to
sit at room temperature with frequent stirring for 60 minutes.
Silicon chips (80.times.80 mil.sup.2) were coated on one side with the
adhesive formulation and placed on a copper lead frame (D/L) and cured in
a 175.degree. C. oven for one hour. The samples were tested after cure for
die shear strength at room temperature (RT). Die shear strength was
measured using a Hybrid Machine Products Corp. die shear tester (Model
1750)/Chatillon DFI 50 digital force gauge. The force required to remove
the bonded die was read in kg units, and converted to a die shear strength
in MPa averaged over ten samples.
Preferred die shear values are those of 13 MPa or greater.
Radius of Curvature
Each epoxy to be tested was prepared, degassed, and aged for one hour as
described above in the die shear test method. Silicon chips (200.times.600
mil.sup.2) were coated with adhesive on one side and placed on a lead
frame and cured in a 175.degree. C. oven for one hour. The radius of
curvature for the cured samples was measured using a Tokyo Seimitsu
SURFCOM surface texture measuring instrument and reported in millimeter
(mm) units, averaged over three samples. Preferred radius of curvature
values are those of 350 mm or greater.
Control Epoxies
The control epoxy resins were the following:
Bisphenol F diglycidyl ether, sold under the tradename EPICLON 830S, a
product of Dai Nippon, having the structure:
##STR14##
Bisphenol A diglycidyl ether, sold under the tradename EPON 828, a product
of Shell Corporation, having the structure:
##STR15##
A dimer acid diglycidyl ester, sold under the tradename EPON 871, a product
of Shell Corporation, having the structure:
##STR16##
A polyglycol diepoxide, sold under the tradename DER 732, a product of Dow
Corporation, having the structure:
##STR17##
Performance Results
Epoxies A-E were tested according to the test methods described above and
compared to the control aromatic epoxy resins and epoxy resins containing
flexibilizers. The results of the flexibility and strength performance
tests, and comparisons to the control epoxies are presented in the
following table. The inventive flexible epoxies demonstrated a low modulus
and high percent strain compared to the controls. Notably, a few of the
flexible epoxies yielded during the strain testing, in contrast to the
controls, all of which snapped.
All of the inventive epoxies and the aromatic control epoxies showed a high
adhesive strength, and in some cases very high radius of curvature. The
EPON 871 epoxy resin and DER 732 epoxy resin controls, which are based on
reactive flexibilizers, do present high radius of curvature values, but
suffer from low die shear strengths.
The control blend of EPON 871 epoxy resin (a reactive flexibilizer) and the
aromatic Bisphenol F epoxide shows good die shear strength, but this is
achieved at the expense of a loss of flexibility as indicated by the
radius of curvature value.
__________________________________________________________________________
Die
Epoxy Cure Modulus Tg .degree.C.
Shear
ROC
Compounds Agent
(GPa)
% Strain
(Tan .delta.)
(MPa)
(mm)
__________________________________________________________________________
Epoxy A a 1.72 5.5 67 21.6 348
(+/-0.28)
yielded
Epoxy B a 3.07 6.4 110 29.8 266
(+/-0.10)
Epoxy C a 0.37 8.3 54 22.5 377
yielded
Epoxy D b -- -- -- 19.7 698
Epoxy E b -- -- -- 19.9 493
Controls
Bisphenol F diglycidyl
a 3.10 5.4 151 26.8 213
ether (+/-0.07)
Bisphenol A diglycidyl
a 2.69 3.9 174 31.8 324
(+/-0.01)
ether
EPON 871/Bisphenol F
a 17.3 288
diglycidyl ether
70:30
EPON 871 5.2 1354
DER 732 1.4 1682
__________________________________________________________________________
Cure catalyst:
a = 2ethyl-4-methyl imidazole, 4-6 pphr (parts per hundred parts resin);
cure
b = phenol novolac (HRJ1166 from Schenectady Chemical), 10 pphr and
2ethyl-4-methyl imidazole, 4 pphr
Example IV
Snap-cured Adhesive Formulations
Inventive epoxies C, D, and E, and a proprietary flexible epoxy, designated
EPOXY X, were formulated into die attach adhesives comprising by weight 80
parts of flexible epoxy and 20 parts aromatic O-glycidyl ether, and 20 or
40 parts per hundred parts of epoxy resin (pphr) of a phenolic hardener.
The curing agent was 5 pphr of 2-ethyl-4-methyl imidazole.
EPOXY X is a proprietary flexible diepoxy synthesized from dimer acid and
having a 36 carbon chain between two epoxy end groups.
The aromatic epoxies were:
A bisphenol-F epoxy resin, sold under the tradename EPON 862, a product of
Shell Chemical Company, having the structure:
##STR18##
A proprietary epoxy resin, designated EPOXY Y, having the structure:
##STR19##
The phenolic hardener was a phenolic novolak resin sold under the tradename
HRJ-1166 from Schenectady Chemical having the structure:
##STR20##
Preparation of the formulations was as follows: the phenolic hardener resin
was dissolved at 130.degree. C. in the flexible epoxy; the imidazole
catalyst was dissolved in the aromatic epoxy; these two mixtures were
blended and the blend mixed with 25% by volume silver flakes.
These formulations, and a control die attach similar to those commercially
available, were used to bond 80.times.80 mil.sup.2 and 200.times.600
mil.sup.2 silicon dies to copper lead frames. These assemblies were
snap-cured on a hot plate at 225.degree. C. for one minute, and were fully
cured as determined by DSC analysis.
The adhesive formulations were tested for die shear strength and radius of
curvature as described in Example III. The die shear tests were run on the
80.times.80 mil.sup.2 die after cure (i) at room temperature (RT), (ii) at
250.degree. C. after heating at 250.degree. C. for one minute (250.degree.
C./1 min) (to simulate the wire bonding operation during manufacture), and
(iii) after boiling in water for two hours, and then at 250.degree. C.
after heating at 250.degree. C. for one minute (100.degree. C. H.sub.2 O;
250.degree. C./1 min) (intended to test resistance to moisture and the
phenomenon known as "pop-corn," a mechanical failure of fully encapsulated
packages after moisture exposure during solder reflow process,
220.degree.-260.degree. C.).
Results are presented in the following table and show that the snap-cured
formulations containing the inventive flexible epoxies maintain their
flexibility and good die shear strength at room temperature and after
being thermally shocked. The die shear strength after exposure to moisture
and thermal shock are within a range acceptable for use as die attach
adhesives.
__________________________________________________________________________
SNAP CURE DIE SHEAR (MPa) RADIUS OF
FORMULATION 100.degree. C.
CURVATURE
Composition
Epoxy Pro- 250.degree. C., 1
H.sub.2 O, 2 hr
250.degree. C./1
parts by - weight
perties
RT min 250.degree. C., 1 min
RT min
__________________________________________________________________________
EPOXY C Tg = 60.degree. C.
23.7 1.4 1.1 420
350 mm
80 WPE = 289
cohesive
cohesive
cohesive
mm
EPON 862
V = 9100
failure
failure
failure
20 cps
Novolak
20
EPOXY D Tg = 52.degree. C.
14.3 1.4 1.2 480
420 mm
80 WPE = 252
cohesive
cohesive
cohesive
mm
EPON 862
V = 410 cps
failure
failure
failure
20
Novolak
40
EPOXY E Tg = 60.degree. C.
23.7 1.5 1.2 400
380 mm
80 WPE = 241
cohesive
cohesive
cohesive
mm
EPON 862
V = 347 cps
failure
failure
failure
20
Novolak
40
FLEX EPOXY
Tg = 28.degree. C.
17.7 1.2 0.8 550
470 mm
X 80 WPE = 376
cohesive
cohesive
cohesive
mm
EPON 862
V = 361 cps
failure
failure
failure
20
Novolak
40
FLEX EPOXY
Tg = 28.degree. C.
14.3 2.0 320
320 mm
X 80 WPE = 376
cohesive
cohesive mm
AROM V = 361 cps
failure
failure
EPOXY Y 20
Novolak
40
Control 23.7 2.1 0.2 230
220 mm
mm
__________________________________________________________________________
Notes:
(a) The epoxy equivalent weight (WPE, weight per epoxy) for each of the
epoxy products was determined by dissolving the compound in glacial aceti
acid followed by titration with standardized HBr/acetic acid (.about.0.1N
to the violet/green transition of crystal violet indicator.
(b) The viscosity of each of the samples was determined using a Brookfiel
conen-plate viscometer at multiple spindle speeds at 25.degree. C. and is
reported as an average of three readings.
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